You can stick a small spring into the hole in the trem block (Fender sells one) that will press the arm against the threads, holding it steady. You also can just wrap PTFE tape around the arm's threads to make the fit tighter. That's what I do. You have to reapply the tape every so often, but whatever.

The arm on my strat did that also.i tried threading it in one more revolution and stripped the threads..I had another mystery arm kicking around that I found in the case of a used guitar I had bought but wasn't for that guitar.it was thicker with bigger/longer threads.with a little bit of persuasion,I was able to re-thread it in to the stock block.fits nicer/tighter than the stock arm ever did and has a more comfortable angle..kinda lucked out on that one..

All the ideas above obviously work for the user's but there is a large variation in the wobble from bridge to bridge, try them all and do what works best on your bridge. The reason I surgest the Teflon tape is it has the smoothest feel and works on all bridges.